1. Field of the Invention
The present invention relates to high-frequency signal transmission lines and electronic apparatuses, and, more particularly, to a high-frequency signal transmission line including a signal line and ground conductors facing the signal line and an electronic apparatus.
2. Description of the Related Art
As a high-frequency transmission line for connecting high-frequency circuits, a coaxial cable is generally used. Since coaxial cables are easily deformed (bent) and are inexpensive, they are widely used.
In recent years, high-frequency apparatuses such as mobile communication terminals have been miniaturized. Accordingly, in such a high-frequency apparatus, it is difficult to obtain the space required for a coaxial cable having a circular cross section. A high-frequency signal transmission line obtained by forming a strip line in a flexible laminate is therefore sometimes used.
Triplate strip lines have a structure in which a signal line is sandwiched between ground conductors. Since the thickness of such a high-frequency signal transmission line in a lamination direction is smaller than the diameters of coaxial cables, the high-frequency signal transmission line can fit into a small space into which the coaxial cables cannot fit.
As an invention related to the high-frequency signal transmission line, a flexible substrate disclosed in Japanese Unexamined Patent Application Publication No. 2007-123740 is known. In the flexible substrate disclosed in Japanese Unexamined Patent Application Publication No. 2007-123740, since a ground conductor has an opening portion, a strip line can be more easily bent as compared with a strip line sandwiched between ground conductors formed on the entire surfaces.
However, in the flexible substrate disclosed in Japanese Unexamined Patent Application Publication No. 2007-123740, an electromagnetic field may exit from the opening portion to the outside of the flexible substrate. In a case where an article, such as a dielectric or a metal body, is disposed around the flexible substrate, electromagnetic field coupling occurs between the signal line in the flexible substrate and the article. As a result, the characteristic impedance of the signal line in the flexible substrate may deviate from a predetermined characteristic impedance.